1. Field of the Invention
The invention relates to a spindle motor, especially for an optical disc drive, capable of inhibiting the Half-omega whirl induced during operation.
2. Description of the Prior Art
One source of vibration in a disc drive is from the spindle motors that they employ. These spindle motors typically include a stator comprising a core having windings arranged thereabout and a rotor shaft. Bearings support the rotor shaft in the radial and axial directions, the bearings being lubricated by impregnated or externally supplied fluid lubricant. One particularly common vibration mode occurs at approximately half the shaft rotation frequency due to the bearings dynamic instability. This vibration mode is called Half-Omega whirl (HOW). This Half-Omega whirl phenomenon is especially prominent in motors that use fluid dynamic bearings while spinning at very low speed.
The traditional solution to this problem includes the following: (A) controlling the gap between the bearing and the shaft to reduce HOW; (B) using other bearing system such as fluid-dynamic bearing system; (C) using some feedback and active control system; and (D) using some preload force to remove the instability by some other external applied dominant preload force.
However, for solution (A), the smaller gap causes some side effect such as higher friction. The bearing system in solution (B) costs higher and the solution (C) isn't applicable for all types of bearing system.
U.S. Pat. No. 5,036,235 discloses a brushless DC motor having stable hydrodynamic bearing system, which has rotational axis of rotor eccentric and parallel to centre line of stator core. This prior art uses a preload force to remove the instability without requiring additional parts and/or labor. But the eccentric stator core would cause a torque variation during rotation by imbalanced magnetic air gap.
Accordingly, an objective of the invention is to provide a spindle motor capable of inhibiting the Half-omega whirl induced during operation.